The invention relates to optical methods and means for determining and/or verifying the geometric dimensions of precision-engineered parts.
The fabrication of precision-engineered components is governed by standard practice in geometric dimensioning and tolerancing (GDandT). Metrology for GDandT requires accurate determination of surface form as well as relationships between part surfaces. The metrology must be accurate and conform to international standards, and preferably takes no more than a few seconds. Surface forms of interest include, for example, opposing plane parallel surfaces, orthogonal plane surfaces, disconnected planar, cylindrical and spherical surfaces, and component surfaces of an assembly.
The invention features optical systems and methods that determine absolute positions of points on potentially disconnected surfaces on a test part with respect to a common reference frame. Such systems and methods allow a user to verify, for example, that the location, relative orientation, and form of part features conform to specifications.
The invention includes an optical system having one or more optical profilers adapted to view a test part from different perspectives. Each profiler is capable of measuring absolute positions of surface points in three dimensions with respect to a coordinate system local to each profiler. The invention further includes initialization and calibration procedures to relate the coordinate systems of each profiler to the other, so as to relate each measured surface position to all others. Such procedures can employ, for example, mechanical standard artifacts or a distance measuring laser interferometer, to provide information regarding the separation and relative orientation of the two corresponding optical profiler coordinate systems. Suitable optical profilers include triangulation systems, time of flight systems, and optical interferometers, such as height-scanning interferometers, which employ mechanical or equivalent scans perpendicular to the surface to obtain a localized (e.g. coherence-limited) interference pattern for each image pixel. As described in greater detail below, height-scanning interferometers that employ infrared sources can be particularly advantageous when working with test parts having relatively rough surfaces.
In general, in one aspect, the invention features a method for determining a geometric property of a test object. The method includes: interferometrically profiling a first surface of the test object with respect to a first datum surface; interferometrically profiling a second surface of the test object in a second coordinate system with respect a second datum surface different from the first datum surface; providing a spatial relationship between the first and second datum surfaces; and calculating the geometric property based on the interferometrically profiled surfaces and the spatial relationship between the first and second datum surfaces.
Embodiments of the method can include any of the following features.
The interferometric profiling of the first surface can provide a distance to each of a plurality points on the first surface from a corresponding point on the first datum surface. Similarly, the interferometric profiling of the second surface can provide a distance to each of a plurality points on the first surface from a corresponding point on the first datum surface.
One or both of the datum surfaces can be a portion of a plane, a curved surface, or have a structured profile.
The first surface of the test object can be spaced from the second surface. The first and second surfaces can correspond to opposite faces of the test object. The first and second surfaces can correspond to adjacent faces of the test object. The first and second surfaces can be adjacent faces separated by a step height. The first and second surfaces can be displaced from one another by a distance greater than a range of the interferometric profiling of the first surface and greater than a range of the interferometric profiling of the second surface.
The interferometric profiling of the first surface can include directing electromagnetic radiation to the first surface along a first direction and the interferometric profiling of the second surface includes directing electromagnetic radiation to the second surface along a second direction different from the first direction. The interferometric profiling of the first surface can include positioning the test object relative to an interferometry system and the interferometric profiling of the second surface includes repositioning the test object relative to at least one component of the interferometry system. For example, the repositioning of the test object relative to the interferometry system can include moving the test object or moving the at least one component of the interferometry system. In the latter case, the method can further include measuring the movement of the at least one component of the interferometry system to determine the spatial relationship between the first and second datum surfaces.
The relationship between the first and second datum surfaces can be defined by a distance between corresponding reference points on the first and second datum surfaces and two angles defining a relative orientation of the first and second datum surfaces.
The method can further including determining the spatial relationship between the first and second datum surfaces.
For example, determining the relationship between the first and second coordinate system can include: interferometrically profiling a first surface of a reference object with respect to the first datum surface; interferometrically profiling a second surface of the reference object with respect to the second coordinate system; providing at least one calibrated dimension for the reference object; and calculating the spatial relationship between the first and second datum surfaces based on the profiled surfaces and the at least one calibrated dimension. The reference object can be selected according to approximate dimensions of the test object.
The method can further including determining the spatial relationship between the first and second datum surfaces based on at least one interferometric displacement measurement. For example, the spatial relationship can be determined based on the at least one interferometric distance measurement and an initial calibration. The method can further include adjusting at least one of the first and second datum surfaces to accommodate the interferometric profiling of the first and second surfaces of the test object and interferometrically measuring the adjustment of the at least one of the first and second datum surfaces to determine the spatial relationship between the first and second datum surfaces.
The spatial relationship can also be determined by: interferometrically profiling a first surface of a initialization artifact with respect to the first datum surface; interferometrically profiling a second surface of the initialization artifact with respect to the second datum surface; calculating an initial spatial relationship between the first and second datum surfaces based on at least the profiled surfaces of the initialization artifact; adjusting the first and second datum surfaces to accommodate the first and second surfaces of the test object, and interferometrically measuring at least one displacement corresponding to the adjustment of the first and second datum surfaces. For example, the first and second surfaces of the initialization artifact can be the front and back of a common interface. Furthermore, the method can provide at least one calibrated dimension for the initialization artifact, and the calculation of the initial relationship can be based on the profiled surfaces of the initialization artifact and the at least one calibrated dimension.
The geometric property can be any of: flatness of the test object; thickness of the test object; parallelism of the test object; a step height; the angular orientation of the first surface relative to the second surfaces (e.g., perpendicularity of the first and second surfaces); and roundness of the test object. Often, the geometric property is defined by positions in a common coordinate system of a plurality of points on the first surface and a plurality of points on the second surface.
The interferometric profiling of at least one of the first and second surfaces can include any of: performing scanning, white light interferometry; performing infrared, scanning interferometry; performing scanning MESA interferometry; performing scanning, (grazing-incidence interferometry; and performing multiple wavelength interferometry.
The test object can also be partially transparent and the geometric property can relate to the positions of points on opposite sides of the partially transparent test object. The first and second surfaces of the test object can be interferometrically profiled from a common side and the first and second datum surfaces can be spaced from one another by a distance greater than a profiling range xcex7 of an interferometry system used for the interferometric profiling steps. Furthermore, the interferometry system used for the interferometric profiling steps can include a reference object having a partially reflective, first surface and a reflective, second surface, the first surface defining the first datum surface and the second surface defining the second datum surface. The spatial relationship can be defined by the spatial separation between the first and second surfaces of the reference object.
In general, in another aspect, the invention features an apparatus for determining a geometric property of a test object. The apparatus includes: means for interferometrically profiling a first surface of the test object with respect to a first datum surface; means for interferometrically profiling a second surface of the test object with respect to a second datum surface different from the first datum system; and means for calculating the geometric property based on the interferometrically profiled surfaces and a spatial relationship between the first and second datum surfaces. The apparatus can further include means for determining the spatial relationship between the first and second datum surfaces.
In general, in another aspect, the invention features an apparatus for determining a geometric property of a test object. The method includes: an interferometric profiling system which during operation interferometrically profiles a first surface of the test object with respect to a first datum surface and interferometrically profiles a second surface of the test object with respect to a second datum surface different from the first datum surface; and an electronic processor coupled to the interferometric profiling system, wherein during operation the electronic processor calculates the geometric property based on the interferometrically profiled surfaces and a spatial relationship between the first and second datum surfaces.
Embodiments of the apparatus can include any of the features described above relating to the method as well as any of the following features.
The interferometric profiling system can include a mount for supporting the test object, wherein the mount is adjustable between a first position for exposing the first surface of the test object and defining the datum surface and a second position for exposing the second surface of the test object and defining the second datum surface.
The interferometric profiling system can includes an interferometric optical profiler having a first viewing port for viewing the first surface of the test object and a second viewing port for viewing the second surface of the test object. For example, the optical profiler can include a first camera positioned to record a field of view for the first viewing port and a second camera positioned to record a field of view for the second viewing port. Alternatively, the optical profiler can include a camera positioned to record a split field of view for the first and second viewing ports. The optical profiler can include at least one source of EM radiation. The optical profiler can further include a first optic positioned to direct a first portion of the EM radiation towards the first viewing port and a second portion of the EM radiation towards the second viewing ports. For example, the first optic can be a beam splitting optic positioned to reflect the first portion of the EM radiation towards the first viewing port, reflect the second portion of the EM radiation towards the second viewing port, and transmit at least one additional portion of the incident EM radiation through the beam splitting optic. The optical profiler can further include a reflective reference surface positioned to receive the at least one additional portion of the incident EM radiation transmitted through the beam splitting optic. Furthermore, the optical profiler can include a transducer coupled to the reflective reference surface for scanning the position of the reflective reference surface.
The optical profiler can include a first viewing port optic supported by a first movable stage, the first viewing port optic positioned to direct at least one part of the first portion of the EM radiation towards the first surface of the test object and the first movable stage adjustable to accommodate the interferometric profiling of the first surface of the test object. Furthermore, the optical profiler can include a second viewing port optic supported by a second movable stage, the second viewing port optic positioned to direct at least one part of the second portion of the EM radiation towards the second surface of the test object and the second movable stage adjustable to accommodate the interferometric profiling of the second surface of the test object. For example, the first reflective optic can be a roof mirror. The optical profiler can further includes a first fold mirror for further directing the at least one part of the first portion of the EM radiation towards the first surface of the test object. The apparatus can also include a displacement measuring interferometer positioned to measure changes in the spatial relationship between the first and second datum surfaces caused by at least one of an adjustment to the first movable stage and an adjustment to the second movable stage.
The interferometric profiling system can include a first interferometric optical profiler for viewing the first surface of the test object and a second interferometric optical profiler for viewing the second surface of the test object. For example, the first optical profiler can be movable relative to the second optical profiler to adjust the spatial relationship between the first and second datum surfaces. Furthermore, the apparatus can include a displacement measuring interferometer positioned to measure changes in the spatial relationship between the first and second datum surfaces caused by relative movement of the first and second optical profilers.
The interferometric profiling system can also include a moveable stage adjustable from a first position defining the first datum surface to a second position defining the second datum surface.
The apparatus can further include a gauge object having first and second surfaces, the first surface being positioned to be profiled by the interferometric profiling system with respect to the first datum surface and the second surface being positioned to be profiled by the interferometric profiling system with respect to the second datum surface. In such embodiments, the electronic processor can determine the spatial relationship between the first and second datum surfaces based on interferometric profiling measurements of the first and second surfaces of the gauge object provided by the first and second optical profilers and, if necessary, at least one calibrated dimension for the gauge object. Furthermore, in some embodiments, the gauge object can be positioned to be in a field of view of the interferometric profiling system during the interferometric profiling of the test object.
The apparatus can further include a displacement measuring interferometer positioned to measure the spatial relationship between the first and second datum surfaces.
The electronic processor can use at least one value indicative of PCOR dispersion in the interferometric profiling system and the test object to calculate the geometric property.
In general, in another aspect, the invention features an apparatus for determining a geometric property of a test object. The apparatus includes: an interferometric profiling system which during operation interferometrically profiles a first surface of the test object with respect to a first datum surface and interferometrically profiles a second surface of the test object with respect to a second datum surface, wherein the interferometric profiling system includes at least one movable stage for adjusting the position of the first datum surface and the second datum surface; a displacement measuring interferometer positioned to measure a change in a relative position of the first and second datum surface caused by an adjustment to the at least one movable stage; and an electronic processor coupled to the interferometric profiling system and the displacement measuring interferometer, wherein during operation the electronic processor calculates the geometric property based on the interferometrically profiled surfaces and the relative position of the first and second datum surfaces.
Embodiments of this aspect can include any of the following features.
The interferometric profiling system can includes a second moveable stage, and during operation the first-mentioned movable stage adjusts the position of the first datum surface and the second movable stage adjusts the position of the second datum surface. Alternatively, the at least one movable stage can include a first movable stage adjustable from a first position defining the first datum surface to a second position defining the second datum surface.
The displacement measuring interferometer can provide multiple axes of measurement (e.g., 2 or 3). The scanning interferometric profiling system can use infrared or visible wavelengths.
In general, in another aspect, the invention features a method for determining a geometric property of a test object. The method includes: profiling a first surface of the test object in a first coordinate system; profiling a second surface of the test object in a second coordinate system different from the first coordinate system; determining a spatial relationship between the first and second coordinate system based on at least one interferometric distance measurement; and calculating the geometric property based on the profiled surfaces and the relationship between the first and second coordinate system.
In general, in another aspect, the invention features a method for determining a geometric property of a test object. The method includes: interferometrically profiling a first surface of the test object with respect to a first datum surface; interferometrically profiling a second surface of the test object with respect to a second datum surface different from the first datum surface; determining a spatial relationship between the interferometrically profiled surfaces which accounts for PCOR dispersion; and calculating the geometric property based on the interferometrically profiled surfaces and the spatial relationship.
In general, in another aspect, the invention features an optical profiling system including: a broadband source; a scanning interferometer which during operation directs a first wavefront along a reference path including a partially reflective first surface and a reflective second surface and a second wavefront along a measurement path contacting a measurement object, and, after the second wavefront contacts the measurement object, combines the wavefronts to produce an optical interference pattern; a detector producing interference data in response to the optical interference pattern; an electronic processor coupled to the detector for analyzing the interference data; and a scanning controller coupled to the scanning interferometer and the electronic processor, wherein during operation the scanning controller causes the scanning interferometer to adjust the position of the first and second surfaces. In some embodiments, the partially reflective first surface of the scanning interferometer defines a first datum surface and the reflective second surface defines a second datum surface, and during operation the electronic processor calculates a geometric property of the test object based on the interference data and a relationship between the first and second datum surfaces.
Embodiments of the invention include many advantages. For example, they can provide high data density, absolute surface topography maps of two or more surfaces of a test part with respect to a common xyz coordinate system. Such topography maps can be used to verify that the geometric dimensions of precision-engineered parts conform to specified tolerances.
Other aspects, advantages, and features of the invention follow.